In 2005, as George W. Bush began his second term and right-wing Republicans organized the House and Senate, most progressives thought the Grand Old Party was as right-wing as it could get. Even so, Florida's Charlie Crist, Alaska's Lisa Murkowski and Rhode Island's Lincoln Chafee remained Republicans in good standing. This year, all three are running insurgent independent campaigns against a GOP that has folded Ronald Reagan's "big tent." The fact that these three celebrants at Bush's Republican Party are no longer welcome is stark evidence of the most dramatic development of this midterm election season.

The GOP's lurch toward extremism extends far beyond the ideological cleansing of moderate RINOs, "Republicans in Name Only." Ideas once considered beyond the fringe could have high-profile champions in the next Congress, and absurd premises and proposals are likely to have their own caucuses. The vast majority of the GOP Senate candidates, for example, reject the overwhelming scientific consensus on the human causes of climate change. Joe Miller, running strong for an Alaska Senate seat against Murkowski, supports repeal of the Seventeenth Amendment, which provides for the direct election of senators; wants to privatize Social Security; and is against abortion even in cases of rape or incest. Both Miller and Sharron Angle, who is in a tight race to take majority leader Harry Reid's Senate seat, want to eliminate the Education Department. And Angle, like Delaware long shot Christine O'Donnell, seems unaware that the Constitution requires the separation of church and state.

Broadening the debate is fine. But when one party pulls that debate toward extremes that even its most radical leaders have recently rejected, the prospect of political dysfunction, if not explosion, grows exponentially. And Republican leaders, with the party's free-spending corporate underwriters, have supplied the dynamite. After working behind the scenes for months to prevent the nomination of Senate candidate Rand Paul of Kentucky, and after the extremism of Paul's antigovernment positions was revealed (including skepticism about the 1964 Civil Rights Act), Senate minority leader Mitch McConnell now says he consults several times each week with the candidate to assure a Paul victory. So it goes, in race after race, as top Republicans and their allies provide partisan, ideological and financial cover for candidates they once dismissed as unqualified, unsavory or unconscionable.

Whether or not voters like it—and polls suggest most do not—America is saddled with a two-party system. Historically, this has required both parties to keep the middle ground in sight. Some of the earliest critics of McCarthyism in the 1950s were Senator Joe McCarthy's fellow Republicans; and when the John Birch Society made its move within the GOP in the early 1960s, rebukes came not just from mainstream party leaders like Richard Nixon and George Romney but from conservative icons Russell Kirk and William F. Buckley.

No more. In 2010 Republican leaders have abandoned even mainstream conservatives like Murkowski while welcoming the wrecking crew. Their money, and the money of independent "angels" like the Karl Rove–guided Crossroads GPS, the Chamber of Commerce and the billionaire Koch brothers, may buy victories on November 2. No matter what the final tabulation, however, President Obama and Congressional Democrats should make it clear that there will be no compromise with extremism. It must be challenged and defeated.